The present invention relates generally to antenna radomes and methods and products for reducing the attenuating target signature of radomes.
Antenna radomes are provided to physically protect antennas which are located in hostile environments or used in applications, such as airplanes, which necessitate enclosing the antenna. Designers of antenna radomes are confronted with the competing interests of providing sufficient protection for the antenna, while also trying to minimize or eliminate distortion and attenuation of the electromagnetic waves emitted from the antenna as they pass through the radome. These competing interests have lead to many design compromises in conventional radomes. For example, although metals possess good strength characteristics, metals were initially not considered suitable materials for radome walls because they would attenuate and distort the outgoing transmissions to an unacceptable degree. Thus, dielectric materials were used to fabricate radomes.
Moreover, radome designers also have to contend with the problem of detection of the antenna by other electromagnetic devices, e.g., radars. The dielectric layers which allow the outgoing transmissions to pass through with minimal distortion and loss, have the drawback that incoming electromagnetic waves can also pass through the radome in the same way. These incoming electromagnetic waves then contact the antenna and reflect back to a receiving device, giving a relatively large return signal.
Somewhat more recently, frequency selective surfaces have been developed whereby metal layers could be used as radome components. The phrase "frequency selective surface" as it is used throughout this description refers to a surface which is designed to pass electromagnetic waves having a particular operating frequency and block, to the extent any metal or conductive sheet blocks, any other frequencies. One exemplary type of frequency selective surface comprises a metal sheet in which slotted elements of a specific shape and size are formed at periodic intervals. These slotted elements act in a manner analogous to a bandpass filter to allow transmission of electromagnetic waves at the resonant frequency of the enclosed antenna without transmission loss at any incident angle and polarity. Examples of such frequency selective surfaces are disclosed in U.S. Pat. No. 3,789,404 to Munk and U.S. Pat. No. 3,975,738 to Pelton et al., which are incorporated here by reference.
These frequency selective surfaces had the advantage that outgoing transmissions at the design operating frequency were not distorted or attenuated, but incoming waves at any other frequency (typically termed a "threat" frequency) were scattered on the frequency selective surface. The slotted openings of the frequency selective surface and shape of the radome scattered the incoming electromagnetic waves so that the returning signature was diminished. Unfortunately, even this diminished signature is detectable and therefore undesirable.